Toothed belt drives are utilized in a wide range of applications where accurate positioning of a driven element by means of a belt drive is required. Thus, such belt drives are used in operating printing mechanisms of typewriters, computer printers, etc. A number of different forms of such belt drives have been developed in an effort to provide such desired accuracy of positioning. In effecting such accuracy, the backlash between the belt teeth and the pulley teeth is a critical factor. The present invention is concerned with an improved structural configuration of such a toothed belt drive providing improved control of the backlash in such a manner as to afford optimum accuracy in the positioning of the driven element.
The belt tooth configuration may vary from a conventional trapezoidal cross section to one wherein the flank surfaces of the belt tooth are arcuate.
Where the belt teeth have arcuate flanks, it is conventional to provide similar arcuate flanks to the pulley teeth.
It has been conventional to construct the belt and pulley teeth so as to provide a backlash between the teeth so as to be substantially equal over the length of the tooth flanks. Alternatively, it has been conventional to provide an increased backlash clearance at the root of the belt teeth so as to provide a smooth engagement of the belt with the pulley.
Such belts are conventionally formed of elastomeric material and a stretching of the belt occurs at times, such as during start-up of the drive. Under such conditions, the belt tooth surface tends to be frictionally rubbed against the pulley and undesirable wear thereof occurs. This problem is particularly vexatious where the drive systems utilize relatively small diameter pulleys, or where repetitive reversal of the drive operation is effected.
Further, by providing a relatively large backlash, it is difficult to accurately position the driven mechanism, such as the carriage of the printing mechanism, which is installed on the toothed belt under relatively low tension.
While gear and chain drives have been used in the past for such applications, the use of flexible toothed transmission belt drive systems is becoming more common in view of the low noise levels and avoidance of lubrication requirements thereof.
Where clearance is provided between the root of the belt tooth and the tip of the pulley tooth to permit smooth engagement of the toothed belt with the pulley, it is difficult to accurately position the pulley and associated mechanism. One attempted solution to this problem is to place the belt under relatively high tension. This, however, has the disadvantage of subjecting the pulley shaft to substantial forces and requires a greater power in the operation of the drive system. On the other hand, reducing the clearance between the belt tooth and pulley tooth to provide improved accuracy and positioning has been found to cause undesirable increased wear as a result of difficulty in effecting a smooth engagement therebetween in the operation of the drive. Thus, the belt teeth become stabilized only after being fully deformed by their engagement with the pulley teeth and because of the dynamic resilient characteristics of the belt, the belt teeth are substantially removed from the pulley groove before the stable condition occurs.
Thus, there has been a longstanding vexatious problem of effecting smooth engagement and disengagement of the belt teeth relative to the pulley teeth, while yet assuring accurate positioning of the pulley by elimination of undesirable clearance therebetween, permitting backlash between the belt and pulley.